问题：The Legacy Build System will be removed in a future release. You can configure the selected build system and this deprecation message in File > Workspace Settings.

解决方案：

Xcode --file--Workspace Settings 

调整完之后，注意最下面一行 “Do not show a diagnostic issue about build system deprecation”，前面的框要勾选☑️上。

完毕之后，问题解决！